Diagnosis and Screening: Principles, methods, applications and organisation of screening for early detection, prevention, treatment and control of disease
Definitions:
Screening is a public health service in which members of a defined population, who do not necessarily perceive they are at risk of, or are already affected by a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications (UK National Screening Committee http://www.nsc.nhs.uk).
Or
Screening refers to the use of simple tests across an apparently healthy population in order to identify individuals who have risk factors or early stages of disease, but do not yet have symptoms (WHO).
Principles:
The underlying concept of screening is that early detection of risk factors or early disease is beneficial for the clinical or public health outcome. Box 3.1.1 below outlines the classic Wilson and Junger criteria formulated to assess whether a condition potentially warrants screening efforts.
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Box 3.1.1 Wilson and Jungner classic screening criteria, WHO 1968
Wilson . JMG, Jungner G. Principles and practice of screening for disease. Geneva: WHO; 1968. Available from: http://www.who.int/bulletin/volumes/86/4/07-050112BP.pdf |
The key questions to ask when deciding on whether to screen for a health problem are listed in Box 3.1.2.
Box 3.1.2
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The disease |
Is it an important health problem? |
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Is the natural history well understood? |
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Is there a long time between the presence of risk factors/sub-clinical disease to overt disease? |
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Does early intervention improve clinical/public health outcome? |
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Screening test |
Is the test valid (sensitivity and specificity)? |
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Is the test simple, reliable and affordable? |
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Is the test acceptable to patient and staff? |
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Diagnosis and treatment |
Is access to diagnostic facilities available and rapid? |
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Is treatment effective and accessible? |
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Is it cost-effective? |
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Is it sustainable? |
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Does benefit outweigh the harm? |
For the assessment and evaluation of screening programmes see the specific section in this chapter.
Limitations of Screening
Screening has important ethical differences from clinical practice as the health service is targeting apparently healthy people, offering to help individuals to make better informed choices about their health. However, there are risks involved and it is important that people have realistic expectations of what a screening programme can deliver. Whilst screening has the potential to save lives or improve quality of life through early diagnosis of serious conditions, it is not a fool-proof process.
Screening can reduce the risk of developing a condition or its complications but it cannot offer a guarantee of protection. In any screening programme, there is an irreducible minimum of false positive results (wrongly reported as having the condition) and false negative results (wrongly reported as not having the condition). The UK National Screening Committee is increasingly presenting screening as risk reduction to emphasise this point.
(UKNSC, www.nsc.nhs.uk)
Methods, applications and organisation
Screening activities can be organised in formal screening programmes (e.g. breast screening) or opportunistically (e.g. assessment of smoking status during primary care visits). Screening instruments can vary from technological procedures (e.g. radiography or laboratory tests), simple clinical examinations (e.g. blood pressure) to a set of standardised questions (e.g. depression screening in primary care).
Brief overview of current UK screening programmes
For detailed information and updates on current programmes please visit the NHS Library for Health http://www.library.nhs.uk/screening/
Tables A, B, C, D amended from 'Mastering Public Health: A postgraduate guide to examinations and revalidation. Lewis GH, Sheringham J, Kalim K, Crayford, TJB. London: Royal Society of Medicine Press Ltd; 2008'
Table A: Antenatal screening programmes
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Standard |
Screening for the following conditions is offered to all women: |
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Down's syndrome |
The combined test is available to women who request screening in the first trimester and understand the implications of doing so. It is based on combining an ultrasound measurement of nuchal translucency (NT; thickness of fluid at the nape of the fetal neck as assessed on ultrasound), and serum levels of human chorionic gonadotrophin (hCG), pregnancy-associated plasma protein A (PAPP-A) and the woman's age |
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The integrated test provides better performance than the combined test if the woman is prepared to wait until the second trimester for the result. It integrates measurements performed at different times during pregnancy into a single test result. Typically it refers to the integration of NT and PAPP-A in the first trimester with the quadruple test in the second |
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The quadruple test is offered to women who attend for screening in the second trimester, and also forms part of the integrated test as described above. It is a second trimester blood test consisting of measurement of alpha-fetoprotein (AFP), unconjugated oestriol (uE3), hCG, inhibin-A and the woman's age (the triple test does not include inhibin-A). |
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If the pregnancy is deemed to be high risk (>1/250 risk of carrying an affected baby with the risk derived from laboratory markers, maternal age, gestational age, and previous affected pregnancy), then the woman is offered amniocentesis (or chorionic villous sampling [CVS] if <13 weeks' gestation). These are diagnostic tests. |
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Fetal anomaly ultrasound scan |
This is performed at 18−20 weeks' gestation to identify structural anomalies in the fetus. It is not universally performed in Scotland. |
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Other |
Those women who are at high risk may be offered screening for psychiatric illness, Tay−Sachs disease, diabetes. |
Table B: Newborn screening programme
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Heel-prick test |
Used to test for: |
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• Phenylketonuria (PKU) |
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• Congenital hypothyroidism (CHT) |
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• Sickle cell disease and thalassaemia (2005) |
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• Cystic fibrosis (2007) |
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Hearing screening programme |
Oto-acoustic emissions test or attenuated brain-stem audiometry |
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Physical examination |
Offered during first 72 hours of life to detect, among others: |
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• Congenital heart disease |
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• Congenital cataract |
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• Congenital malformation |
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• Cryptorchidism (undescended testes) |
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• Developmental dysplasia of the hip |
Table C: Children's screening programmes
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Growth |
Height and weight should be measured around the time of school entry. The measurements can be used to identify growth disorders, and are an important public health population measurement. |
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Hearing |
Screening for hearing impairment is offered to school-aged children. |
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Vision |
Screening for visual impairment is offered at age 4−5 years. |
Table D: Adult screening programmes
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Disease |
Screening Test |
Confirmatory Test |
Age group |
Frequency |
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Breast cancer |
Fine-needle aspiration |
50−70 (first invitation always before age 53) |
3-yearly |
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Cervical cancer |
In England: • 25−49 years • 50-64 years In Scotland: 20−60 years |
• 3-yearly • 5-yearly • 3-yearly |
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Bowel cancer |
Faecal occult blood |
Colonoscopy |
60−69 |
2-yearly |
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Genital chlamydia trachomatis |
Nucleic acid test (NAT) (taken by genital swab/urine) |
Genitourinary clinic or outreach clinic appointment |
Young men and women |
Opportunistic |
© Dr Murad Ruf and Dr Oliver Morgan 2008
